Dental composite restorative materials are typical dental curable compositions. In a dental clinic, for example, a dental composite restorative material is filled into the cavity of a tooth to be restored, formed into the tooth shape, and then polymerized and cured by irradiation with active light using a special irradiator, so that the damaged tooth is restored.
In a dental laboratory, a dental composite restorative material on a plaster cast is built in the form of a tooth to be restored and then polymerized and cured by light irradiation. In a dental clinic, the product is bonded to the tooth with a dental adhesive, so that the damaged tooth is restored.
Dental composite restorative materials are advantageous in that they can have substantially the same color as that of natural teeth and good handling property. In recent years, therefore, dental composite restorative materials have spread rapidly, and now they have been used in most of the front teeth treatments. Dental composite restorative materials with considerably high mechanical strength have also been developed. As a result, dental composite restorative materials are also beginning to be used in the restoration of posterior teeth, to which high bite pressure is applied.
A dental composite restorative material generally includes a polymerizable monomer (monomer), a filler, and a polymerization initiator as main components. The material, shape, particle size, and content of the filler to be used are selected, when a dental composite restorative material is formed. When they are selected appropriately, various properties such as the handling property of the dental composite restorative paste and the esthetics, mechanical strength, and other properties of the cured product are controlled optimally.
For example, when an inorganic filler with a large particle size is added to a dental composite restorative material, the resulting composite restorative material can form a cured product with high mechanical strength. This is advantageous to the dental composite restorative material. On the other hand, however, the cured product can have reduced surface smoothness or wear resistance. As a result, it may be difficult to obtain a cured product with a glossy finish surface like natural teeth.
A fine inorganic filler with an average particle size of 1 μm or less can also form a cured product with good surface smoothness or wear resistance. However, a fine inorganic filler, which has a large specific surface area, can significantly increase the viscosity of a paste composite restorative material. Meanwhile, in the treatment of teeth, it is necessary for a dentist to adjust the viscosity of a composite restorative material to a level suitable for use in the oral cavity. In order to reduce the viscosity, it is necessary to reduce the content of a fine inorganic filler. In this case, problems may occur, such as a reduction in handling property during treatment, an increase in the shrinkage of a cured product, which is associated with the polymerization of a monomer during the curing of a composite restorative material, and a reduction in the mechanical strength of the resulting cured material.
Under the circumstances, the use of an organic-inorganic composite filler is proposed (see for example Patent Literatures 1 and 2). According to these Patent Literatures, when such an organic-inorganic composite filler is used, a paste composite restorative material with good handling property can be obtained with good surface smoothness and wear resistance maintained as in the case where a fine inorganic filler is used, and the polymerization shrinkage of the cured product is also reduced.
Such an organic-inorganic composite filler includes an organic resin filler and a fine inorganic filler contained therein. The organic-inorganic composite filler has a surface area smaller than that of the fine inorganic filler. Therefore, a sufficient amount of the organic-inorganic composite filler can be added without causing thickening when a paste composite restorative material is manufactured.
A general method of manufacturing the organic-inorganic composite filler includes preliminarily kneading a fine inorganic filler and a polymerizable monomer to form a curable composition, polymerizing the curable composition to form a cured product, and then grinding the cured product (see paragraph [0012] of Patent Literature 1).
There is also known a method for manufacturing an organic-inorganic composite filler with a narrow particle size distribution (see Claims of Patent Literature 2). In this method, inorganic agglomerated particles are first manufactured by a method of granulating a fine inorganic filler, such as spray drying. Subsequently, the manufactured inorganic agglomerated particles, which are in contact with a liquid polymerizable monomer under reduced pressure, are allowed to return to the original pressure, so that the polymerizable monomer is allowed to penetrate the intra-agglomerate voids of primary particles constituting the inorganic agglomerated particles.
Subsequently, the penetrating monomer is polymerized and cured to form an organic-inorganic composite filler. This organic-inorganic composite filler may also be used without being ground.
The Literature states that in the manufacturing method, the polymerizable monomer may be diluted with a volatile solvent when the inorganic agglomerated particles are brought into contact with the polymerizable monomer (paragraphs [0042]-[0043]). The reason is that the polymerizable monomer should be allowed to sufficiently penetrate the intra-agglomerate voids of the inorganic agglomerated particles.
Unfortunately, the description of the Literature is silent on how much the volatile solvent should be used and particularly silent on what process should be used to remove the volatile solvent after the monomer is allowed to penetrate the intra-agglomerate voids. The Literature also discloses dropping or continuous mixing as a method for bringing the polymerizable monomer into contact with the inorganic agglomerated particles, in which such a continuous operation allows as much the polymerizable monomer as possible to penetrate the intra-agglomerate voids of the inorganic agglomerated particles.
From the description of the Literature, therefore, it is apparent that even the mode of diluting the polymerizable monomer with a volatile solvent is no different from the technical idea of charging the polymerizable monomer as much as possible into the intra-agglomerate voids. In other words, it suggests that the volatile solvent should be used in a minimum amount. Therefore, it is considered that in the mode of diluting the polymerizable monomer with a volatile solvent, the dilute solution of the polymerizable monomer is allowed to penetrate the intra-agglomerate voids, while the volatile solvent is evaporated from the penetrating dilute solution. Thus, it is considered that the dilution mode is intended to charge a sufficient amount of the polymerizable monomer into the whole of the intra-agglomerate voids based on the continuation of the penetration and the evaporation before the polymerization and curing.